Clinical significance and biomarker potential of MGMT protein measurement in colorectal cancer.

Abstract

136 Background: O6-methylguanine DNA methyltransferase (MGMT) is the principal repair mechanism of DNA damage created by alkylating agents. Approximately 40% of colorectal cancers (CRC) exhibit MGMT promoter hypermethylation, expected to result in gene silencing. The role of MGMT as a biomarker is poorly understood, and quantitative measurement of MGMT protein has not been performed in CRC. We hypothesized that altered tumor MGMT would affect DNA damage response, modulate adaptive anti-tumor immune responses, and influence survival in CRC. Methods: We used multiplexed quantitative immunofluorescence (QIF) to study 4 clinically annotated retrospective patient cohorts treated from 2000 to 2017 at Yale, consisting of 400 formalin-fixed paraffin-embedded CRC cases represented in tissue microarray format. These included paired tumors and adjacent non-tumor colonic mucosa (n = 112 pairs in Cohort 1), paired primary CRC and lymph node metastases (n = 31 pairs in Cohort 2), and 2 cohorts with primary stage I-IV tumors (n = 250 in Cohort 3; n = 150 in Cohort 4). We established a QIF panel for simultaneous, localized measurement of DAPI (all cells), cytokeratin (CK; tumor epithelial cells), MGMT, γH2AX (DNA damage response), and CD8. The measurement of fluorescent signals in CK-positive tumor cells or CK-negative stromal cells was achieved using co-localization strategies and the AQUA QIF platform. Results: We identified lower MGMT protein levels in CRC cells than in non-tumor colonic epithelium (p < 0.0001), and in lymph node metastases compared to paired primary CRC tumors (p = 0.0411). Using the visual detection threshold, tumor selective MGMT protein downregulation was identified in 20% of cases in Cohorts 3 and 4. Microsatellite instability-high (MSI-H) cases showed decreased tumoral MGMT protein compared to microsatellite stable (MSS) cases (p = 0.002). In Cohorts 3 and 4, low tumoral MGMT was consistently associated with increased CD8+ tumor infiltrating lymphocytes (p = 0.0472; p = 0.0002). The association between MGMT and γH2AX was inconsistent. The association between MGMT and CD8 was significant only in MSS cases (p = 0.0001) but not in MSI-H cases (p = 0.0979), supporting that the effect is not driven by MSI-H status. Tumor-selective MGMT downregulation evidenced by a low tumor-to-stroma ratio was associated with improved progression-free survival and overall survival in both Cohorts 3 and 4, but statistical significance was only seen in Cohort 3. Conclusions: MGMT protein downregulation occurs in 20% of CRCs and is associated with increased adaptive anti-tumor immune responses, mismatch repair (MMR) deficiency and better prognosis. MGMT deficiency may alter DNA repair in tumor cells and mediate the accumulation of antigenic mutations or neopeptides, independent from MMR status. Our results support a biomarker role of MGMT protein and suggest a role for immunotherapy combinations in MGMT deficient tumors.